The use of builtins.isStorePath can cause evaluation to diverge between Nix expressions in the Nix store versus those outside, see e.g. NixOS/nixpkgs#153594 (comment). Perhaps builtins.isStorePath should return false for flake source paths, even if they happen to be stored in the Nix store (which is arguably an implementation detail). In fact this will probably happen due to #3121 because we won't copy flakes to the store unless required.